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HCM can be defined broadly as the presence of increased LV wall thickness that is not solely explained by abnormal loading conditions.1 More specifically, HCM is a genetically determined disease wherein sarcomere mutations lead to LV hypertrophy associated with nondilated ventricular chambers in the absence of another disease that would be capable of causing the magnitude of hypertrophy present in a given individual.2
- HCM is the most commonly inherited heart defect, occurring in 1 out of 500 individuals.
- Approximately 500,000 people have HCM in the United States, although many are unaware. An estimated 36% of young athletes who die suddenly have probable or definite HCM, making it the leading cause of SCD in young people in the United States, including trained athletes.3
- HCM results from a mutation in a gene encoding one of the proteins involved in essential myocardial sarcomere functions, such as structural or contractile proteins, calcium handling proteins, or mitochondrial proteins.
- There are currently at least 30 HCM susceptibility genes that lead to autosomal dominant transmission with variable phenotypic expression and penetrance.4,5,6
- The most common mutations involve myosin binding protein C (MYBPC3) and myosin heavy chain 7 (MYH7).
- Over 50% of clinically affected patients have an identified mutation.7
- The histopathologic change in HCM consists of hypertrophied myocytes arranged in a disorganized manner with interstitial fibrosis.
- These changes lead grossly to myocardial hypertrophy that is typically predominant in the ventricular septum (asymmetric septal hypertrophy) but may involve any and all ventricular segments. HCM can be classified clinically according to the presence or absence of LV outflow tract (LVOT) obstruction. When present, it is termed hypertrophic obstructive cardiomyopathy (HOCM).
- LVOT obstruction may occur at rest but is enhanced by factors that increase LV contractility (exercise), decrease ventricular volume (e.g., Valsalva maneuver, volume depletion, large meal), or decrease afterload (vasodilators).
- Delayed ventricular diastolic relaxation and decreased compliance are common and, along with MR, may lead to pulmonary congestion.
- Myocardial ischemia is common, secondary to a myocardial oxygen supply–demand mismatch.
- Systolic anterior motion (SAM) of the anterior leaflet of the mitral valve is often associated with MR and likely determines the severity of LVOT obstruction.